During the last year we have demonstrated that the cardiac microsomes isolated from hearts of conditioned rats have an increased rate of binding for calcium. During the coming year, we plan to investigate further the mechanism of this increased calcium control as an adaptation to physical conditioning. We have demonstrated that the increase in actomyosin ATPase activity of hearts of physically trained animals is not maintained unless a physical training program is continued. During the current year we will further investigate different exercise programs on the actomyosin ATPase activity. We have demonstrated that one of the mechanisms for increased myosin ATPase activity in hearts of conditioned rats relates to sulfhydryl group availability and control at or near the active site. We have demonstrated also that the sulfhydryl groups involved in this control are those that bind adenosine di- and triphsophates. Preliminary studies suggest that the increased vascularity observed in hearts of conditioned rats confers a greater coronary reserve when coronary flow is measured by microsphere technique. During the coming years we will conclude a number of the above sections and initiate studies on the myocardial mechanics of physically trained hearts as measured in the isolated working rat heart apparatus. We will also initiate studies of effects of physical training upon myocardial protein turnover. Bibliographic references: Bhan, A.K. and Scheuer, J.: The Effects of Physical Training on Cardiac Myosin ATPase Activity. American Journal of Physiology 228: 1178-1182, 1975. Bhan, A. K., Malhotra, A., and Scheuer, J.: Biochemical Adaptations in Cardiac Muscle: Effects of Physical Training on Regulatory Sulfhydryl Groups of Myosin. Journal of Molecular and Cellular Cardiology 7: 435-442, 1975.